When should you set the altimeter after receiving QNH values from ATC?

Setting the altimeter to the QNH values provided by Air Traffic Control is crucial for maintaining accurate altitude references, especially during the transition from flight levels to altitudes. The correct moment to adjust the altimeter is when passing the transition level (TLv).

The transition level is the altitude determined by ATC, above which aircraft are expected to operate in terms of flight levels (using the standard altimeter setting of 29.92 inches of mercury) and below which altimeters must be set to QNH for precise altitude reporting. By setting the altimeter to the QNH values as you pass the TLv, you ensure that you are properly aligned with the defined altitudes in the controlled airspace, contributing to safety and situational awareness.

Adjusting the altimeter at this point helps in preventing altitude discrepancies between you and other aircraft, especially as you climb to or descend from the transition level. This timing is crucial for safe operations when interacting with other traffic and ATC instructions.